The present invention is directed to soot free autothermal reforming (ATR) of hydrocarbon feed.
In the autothermal reforming, combustion of hydrocarbon feed is carried out with substoichiometric amounts of oxygen by flame reactions in a burner combustion zone and, subsequently, steam reforming of the partially combusted feedstock in a fixed bed of steam reforming catalyst. Substoichiometric combustion of hydrocarbons leads disadvantageously to formation of soot. Soot formation may be avoided by using a specific burner design and through controlling the operating conditions of the ATR process. Soot is formed in the flame of an autothermal reactor within certain ranges of operating conditions. When the amount of steam relative to the other components send to the ATR reactor is under a critical value, soot is formed in the reacting feed. The design of the burner nozzles has influence on the critical steam to carbon ratio. One such burner useful in ATR is described in U.S. Pat. No. 5,496,170. The limiting amount of steam can be expressed as the critical steam to carbon ratio, which is the molar feed flow rate of steam to the molar flow rate of carbon in the hydrocarbon feed. The hydrocarbon feedstocks can be in form of natural gas or another kind of hydrocarbon including LPG, butane, naphtha, etc. The molar flow rate of carbon is calculated as the molar flow rate of the hydrocarbon times the carbon contents of the hydrocarbon.
Examples of operation conditions, which do not result in soot formation, are summarized in a paper by Christensen and Primdahl (Hydrocarbon Processing, March 1994, pages 39-46). Those conditions are shown in Table 1. The tests have been conducted in a pilot plant. Due to heat loss from the relatively small pilot unit, the adiabatic ATR exit temperature will be higher than the measured ATR exit temperature. This means that if a large unit, from which the heat loss is negligible, is subjected to the exact same operating conditions, the ATR exit temperature will be close to the adiabatic ATR exit temperature. The soot precursors are formed in the combustion zone of the ATR. Most of the heat loss occurs after the combustion zone. A subsequent heat loss cannot have any influence on the reactions in the combustion zone. The oxygen to carbon ratio (O.sub.2 /C) is also shown in Table 1. The definition of this ratio is analogue to the steam to carbon ratio, however, with steam substituted by oxygen. The exit temperature from the ATR reactor can be calculated from the O.sub.2 /C ratio, when the heat loss from the reactor is known.
TABLE 1 ______________________________________ Oxygen to Measured Adiabatic Case Carbon ATR Exit ATR Exit No. Ratio H.sub.2 O/C CO.sub.2 /C Temp..degree. C. Temp..degree. C. ______________________________________ A 0.60 1.43 0 950 1013 B 0.62 0.59 0 1065 1173 C 0.60 0.86 0 1000 1073 D 0.67 0.68 0.47 1018 1147 E 0.70 0.67 0.75 1030 1147 F 0.73 0.58 0.98 1028 1177 ______________________________________
Operation conditions which do not result in soot formation (from Christensen and Primdahl, 1994)
Advantageously, the process is operated at low steam to carbon ratios, since a low ratio lowers the investment expenses for an ATR plant and reduces the necessary energy consumption in operating the plant. Additionally, a low steam to carbon ratio makes it possible to optimize the produced synthesis gas composition for production of CO-rich gases for e.g. methanol or dimethyl ether synthesis and Fischer-Tropsh processes.
It has been found that the operating pressure has a very strong influence of the critical steam to carbon ratio.
It has further been found that at a given temperature and steam to carbon ratio in the feedstock, the feedstock is autothermally reformed without formation of detrimental soot when adjusting the operation pressure in the autothermal reactor above a critical value.